1. Field of the Invention
The present invention relates to vector coding, and more particularly, to a method for coding a motion vector in a moving picture.
2. Background of the Related Art
An outline of an emerging Moving Picture Expert Group (MPEG) standardization MPEG-4 will now be described. The MPEG-4 pursues a technique different from the techniques of MPEG-1 and MPEG-2 previously used to cope with the current trend in which boundaries of techniques of communication, computer and broadcasting are removed and transformed into multimedia. That is, the object of MPEG-4 is a comprehensive technique that embraces each field of the above techniques and their independent technical innovations independently as well as the exploration of fields to which the new comprehensive technique is applicable. The directions that MPEG-4 pursues at large is a technique that allows an Audio and Video (AV) coding at an extra low bit rate.
To realize the technique that allows an AV coding at an extra low bit rate, the following techniques are suggested. First, a new coding technique in which the MPEG-1 and MPEG-2 DCT transform technique is not used, for example, the Wavelet coding system that has less distortion at block boundaries and a high coding efficiency. Second, a technique that conducts motion compensation in a manner other than the MPEG-1 and MPEG-2 block based motion compensation, which conducts the motion compensation by a macro block unit. Third, a region based coding technique inclusive of the contour coding technique and the object oriented coding technique. Fourth, a Fractal coding technique that can make a compression ratio over 1/1000.
Further, there are 8 new techniques that are not yet realized but are trying to be fully realized in MPEG-4. The eight new techniques include improvement of a coding efficiency, scaleability matching to a content, content handling and bit stream editing, improvement of tolerance for error, access to a multimedia data base, coding of a plurality of synchronous data, hybrid coding of natural data and synthesized data, and improvement of a random accessibility at an extra low bit rate.
A related art method for coding a motion vector will be explained with reference to FIGS. 1-3. FIG. 1 shows a motion vector bound of 8.times.8 motion vectors at an 8.times.8 mode macroblock. FIGS. 2a-2d illustrate definitions of candidate predictors for motion vectors in 8.times.8 mode macroblock. FIG. 3 illustrates an example of the case when a predicted motion vector median value lies outside of a bound.
In the emerging MPEG-4 standard, the 8.times.8 block search operation for integer pixel motion estimation is conducted within .+-.2 pixel search window centered on a 16.times.16 motion vector. Therefore, if a particular macroblock is searched in the 8.times.8 mode, all the motion vectors for 4 blocks in the macroblock will lie within a bound. The matrix shown in FIG. 1 shows the bound within which the 4 block motion vectors in the macroblock searched in the 8.times.8 mode can exist. This bound for the 4 motion vectors serves to improve a coding efficiency of motion vectors. Each of the motion vectors representing the 4 blocks in the bound is estimated as median values of neighboring three motion vector candidates, and the x and y components of the difference between the true and the estimated motion vectors, MVD.sub.X and MVD.sub.Y, are variable-length coded, which is expressed as the following. EQU Px=Median(MV1x, MV2x, MV3x), EQU Py=Median(MV1y, MV2y, MV3y),
and EQU MVDx=MVx-Px, MVDy=MVy-Py.
FIGS. 2a-2d illustrate motion vector candidates in a 8.times.8 mode according to an algorithm for estimation of a motion vector in MPEG-4. However, the motion vector estimation bound shown in FIG. 1 can not support an MPEG-4 Verification Model (VM) for the following reasons.
Referring to FIGS. 2a-2d, when the motion vector prediction candidates in the 8.times.8 mode macroblock are reviewed, cases exist when the motion vector prediction candidates of which median values are used for estimation of motion vectors lie outside of the bound of motion vectors in a 8.times.8 mode. Only the case of MV2 of block 2 is outside the bound of motion vectors in a 8.times.8 mode if the case of block 1 is excluded. As shown in FIG. 2b, only one of the three motion vector prediction candidates lies within the 8.times.8 mode macroblock. In this case, an absolute value of the MVD2 obtained from the predicted value of the median values may lie outside of a bound of 5.0. As shown in FIG. 3, when an MV of block 2 is denoted as MV.sub.CurrentBlock (a motion vector for respective block), an MV1 of block 2 is denoted as Mv.sub.insideMBprediction (a motion vector estimation within the bound), and a Median(MV1, MV2, MV3) of block 2 is denoted as MV.sub.MedianPrediction, there is a case when the MV.sub.MedianPrediction lies on a point outside of the bound.
Therefore, the related art method for coding a motion vector has various disadvantages. The related art method for coding a motion vector in a moving picture has a problem of bit loss when an estimated value from medians lies outside of a bound.